Weapons and Size
The statistics given for a weapon assume a Medium wielder, which includes most adventurers. Creatures that are smaller or larger than Medium have special rules.
Small creatures use the same weapons that Medium creatures do. However, a Small creature (such as a halfling) can’t use a two-handed weapon. When a Small character uses a versatile weapon, he or she must wield it two-handed and doesn’t deal any extra damage for doing so.
Large, Huge, and Gargantuan creatures use weapons that are specially sized for them, and a monster’s stat block accounts for how the monster’s size interacts with any weapons it wields.
If it’s necessary to figure out the damage of a larger-than-normal weapon, use this rule of thumb: Each size category larger than Medium increases the weapon’s damage die by one size, as shown below.
1d4 -> 1d6 -> 1d8 -> 1d10 -> 1d12 -> 2d6 -> 2d8 -> 2d10
1d8 -> 2d4 -> 1d10 -> 1d12 -> 2d6 -> 2d8 -> 2d10
Thus, a longsword sized for a Large fire giant deals 1d10 weapon damage instead of 1d8, and a Large quarterstaff deals 2d4 weapon damage instead of 1d8.
Large creatures can use two-handed weapons intended for creatures one size category smaller than themselves and treat the weapons as one-handed weapons. For instance, a fire giant (Large) can use a human’s greatsword with one hand, and a fire titan (Huge) can use a fire giant’s greatsword with one hand. A creature can’t use an undersized one-handed weapon at all; its hand is too large to effectively hold the weapon’s small grip.
Creatures can’t use weapons designed for creatures larger than themselves. A human adventurer can’t fit his or her hands properly around the hilt of a fire giant’s dagger, let alone use it as an effective weapon.
Published in Player's Handbook, page(s) 220, Rules Compendium, page(s) 270.